1. Field of Invention
The present invention relates to a highway lighting system, adaptations to current highway lighting systems, and associated methodology. More specifically, the highway lighting system aspect of the present invention involves converting AC to DC power and removing harmonics centrally in a way to efficiently power and to control individually LED highway lamps. In effect, the highway lighting system aspect of the present invention is able to utilize the methodology aspect of the invention to automatically test whether individual LED lamps are functioning properly. Moreover, the present invention allows for the implementation of new highway installations and also for retrofitting existing highway installations.
2. Discussion of the Prior Art
Highway and freeway street lighting systems usually place a lamp post every 50 m apart. Depending on the required lighting in each particular use, the lamps can be along the middle line of the road or on both sides of the road. While highway and freeway street lighting may appear to be controlled on a single grid, the entire lighting grid is usually split up and controlled in approximately 2 km sections. In any given 2 km section, all the lighting is powered by a single power supply (this is at least in the case in China, Europe, and many other countries). That means for every 2 km section there are 40 lamps posts on the middle line of or both sides of the highway. Depending on the amount of lighting required for the particular application, each lamp post can contain any lighting capacity, i.e., each lamp post could have any number of lights but they are normally turned on and off at the same time in one group.
The standard type of lights used at each lamp has recently changed. High Intensity Discharge (HID) lights, such as High Pressure Sodium and Metal Halide etc. were the mainstream type of light used for highway lighting until around 2010 at which point the standard became Light Emitting Diodes (LED). LED lighting provides at least two main benefits over HID lighting: (a) LED is energy saving, and (b) LED is easier to dim. There are a number of practical differences between using HID and LED in highway lighting. One main difference is that LED lighting requires direct current (DC) of lower voltage, say around 50 V, while HID requires alternating current (AC). This is notable due to the fact that the existing power distribution is in AC. Even if the LED light is energized by a 110 V or 220 V AC, the power needs to be converted to DC at around 50 V or below to be directly fed to the LEDs. Two world standards of AC wiring distribution are common. In Europe, China, Hong Kong and many countries, a three phase AC-4 wire (i.e., three hot AC wires plus a neutral wire) (220/240 V single phase-380/415 V three phase) system is utilized. In North America, the secondary services could be a three phase AC-4 wire (277/480 V), or single phase AC-3 wire (120/240 V) or single phase AC-3 wire (240/480 V) system. No matter what system is adopted, the HID uses a single phase AC, which means at every HID lamp post only two wires go in (e.g., one phase wire and the neutral) regardless of whether the three phase AC or single phase AC distribution is used.
The lighting standard move from HID to LED does not simply require replacing a HID light with a LED light. Rather, because LED uses DC while the existing HID uses AC, the conversion thus requires converting the existing AC to DC. The typical LED lighting alongside highways only requires around 50 V DC, so the converter box at each lamp post has to convert the 120 V, 220 V or 240 V AC to 50 V DC in whatever country. The power supply of existing LED based highway lamps at the lamp post is normally in single phase of alternating current ranging from 220 V in Hong Kong, 240 V in Australia and Britain, and 277 V in the U.S.A. Even though energy is lost during the AC to DC conversion, the energy saved by using LED lighting is still considered worthy.
The energy savings from LED do not come without its disadvantages. Recall that with the conventional HID approach, highway lighting system does not require electronics basically, i.e., there is no controller because HID uses AC as design. For LEDs, substantial electronics consisting of lots of active components is needed for the voltage conversion, rectification and control. Electronics creates unwanted harmonics because they are non-linear active components. There is no possibility of the HID lighting system to create harmonics as basically no electronics is involved. However, introducing LED lighting requires a controller which in turn generates harmonics. And because governments heavily regulate the power quality in terms of Total Power Factor and Harmonics Distortion (i.e., the issue of power quality deals with the amount of harmonics created by the load such as a lamp, controller, or anything that uses electric power), government regulation requires harmonics to be reduced, so the controller in LED lighting must remove some amount of the harmonics (depending on the specific regulations). That the controller at each lamp must regulate the harmonics created, which consumes a certain amount of energy, is a second source of energy consumption leading to less efficiency in modifying HID lighting to LED lighting.
To be concrete, consider the following example to demonstrate the loss of energy in a typical LED lighting system. FIG. 1 provides a diagram of a 2 km section of highway lighting powered by a single 220 V AC power source 10, and containing forty LED lights 20, AC to DC converter 30, and harmonics removal devices 40. Assuming voltage drop away from the power supply 10 is negligible, notice that each of the forty lamps receives an input of 220 V AC from the power source, which is located at the location labeled 31, and must be converted to 50 V DC for the 0.4 kW LED lamps. However, notice that because, for example, 0.1 kW are consumed in each of the controllers (i.e., the controller here consists of an AC to DC converter 30 and harmonics removal device 40, where each lamp and controller consumes 0.5 kW), the LEDs themselves only consumes 0.4 kW. At location 21 below, there is a 50V DC. The total efficiency of the system is 80%, which is found by dividing the energy emitted as light (i.e., the 0.4 kW per lamp multiplied by the 40 lamps equals 16 kW) by the energy consumed (i.e., 0.5 kW per lamp and controller multiplied by 40 lamps equals 20 kW).
One observation should be made regarding FIG. 1. Notice that the diagram assumes each lamp's controller will receive 220 V AC. That is, the voltage supplied to each controller down the line is uniform. Realistically, a voltage drop will occur as the distance away from the power supply increases because wires naturally have some resistance, where the last lamp post will have a slightly lower input voltage. Given that each controller will use the same AC to DC converter and there is no automatic voltage regulation within each controller, the voltage supplied to the lamps will decrease as the distance from the power supply increases. Accordingly, the lamp closest to the power supply will be slightly brighter than the lamp one away from the power supply, and the lamp one away from the power supply will be slightly brighter than the lamp two away from the power supply, and so on, with the lamp furthest away being the dimmest. Directly after the last (dimmest) lamp will come another lamp which is the first lamp supplied by a different power supply, so the dimmest lamp from one power supply will be located next to the brightest lamp of another power supply.
Five more observations deal with the arrangement of the power supply and the controllers. First, notice that one power supply is responsible for all lamps within the 2 km section. Therefore, if the power supply of one 2 km section fails, all lamps will be off until that power supply is fixed. There is no backup power supply that could provide power to the lights while the other power supply is in maintenance.
Second, the power supply simply provides one function: to supply power while the converter inside each controller is only responsible for converting AC to DC. Thus, there is no individual control of each of the 40 or so lamps from the centralized power supply. Without each lamp being individually controlled, there is no way to dim one lamp in a 2 km section of highway without dimming all lamps in the entire 2 km section of the highway. Individual dimming control could be useful for a number of reasons, including to save energy and for safety. For example, the brightness of lamps nearest to off and on ramps, or other breaks in the highway, in construction areas, etc. could be set higher than the brightness of lamps in areas that do not require such high brightness.
The third observation relates to how the entire 2 km section of lighting would be dimmed at the same time, namely, by reducing the power supplied from the power supply. However, due to the placement of AC to DC converter (i.e., at each controller), the setup only allows for limited dimming of up to around 30%. It is because the converter circuit needs at least some voltage to function and therefore the voltage of the general power supply of the whole 2 km section could not get too low or else, the converter will not function at all.
The fourth observation also relates to power supply only supplying power: there is at present an absence of a centralized way to monitor which lamps are functioning properly. At present, human inspectors drive around at night and manually check to see if the lamps are functioning properly (i.e., by checking to see if each light has failed or is flickering) and therefore need to be replaced.
The fifth observation relates to the voltage of power transmission. It can be seen that the single phase AC voltage only goes up to a top limit of 277 V and it could sometimes be down to 120 V. It is well aware that the higher the transmission voltage is, the lower the loss along the transmission line is because the current flowing only the line is lower and thus generating less heat due to cable resistance.
The present invention addresses the need for a more efficient way for LED highway street lighting systems to (a) use a higher voltage of transmission along the 2 km sector, (b) convert AC to DC centrally instead of locally at the lamp post, (c) deal with the harmonics created centrally, (d) transmit power to the lamps, (e) overcoming the non-uniform brightness among the LED lighting created by the slight drop in voltage due to the resistance of the power wires, (f) save energy by being able to dim the LED lights independently of one another while maintain the highway safety standard, (g) inspect whether every LED light is functioning properly, (h) receives remote commands from a central station of the city to turn on/off, dim and check the health of LED lights.